A method for the presulphurization of catalysts used in refining and in petrochemistry. Sulphur is incorporated into the catalytic material and then at the time of starting up the refining or petrochemical reactions, in the presence of hydrogen, the oxides of the active metals present in the catalyst are converted into sulphides.
In methods for the incorporation of sulphur into or unto the catalytic material, a sulphurized compound chosen from among all adequate sulphurized compounds, particularly the organic polysulphides described in the Applicant's U.S. Pat. No. 4,530,917 in accordance with the formula: R--S(n)--R', is incorporated, in which n is an integer from 3 to 20 and where the radicals R and R', which can be the same or different, in each case represent an organic radical, each containing 1 to 150 carbon atoms per molecule, said radicals being chosen from within the group constituted by saturated or unsaturated, straight or branched-chain alkyl radicals or of the naphthene type, aryl radicals, alkyl aryl radicals and aryl alkyl radicals and in which R' can also represent the hydrogen atom, with as an example of the polysulphide ditert. dodecyl polyslphide (n=5) and ditert. nonyl polysulpide (n=5).
It is also possible to use ammonium disulphide and/or sulphur in powder form (flowers of sulphur), the latter then being used in suspension alone or mixed with another sulphurized compound (e.g. an organic polysulphide as defined hereinbefore) and as described in French patent application EU 90/03596 of Mar. 19, 1990.
In the method for the starting of petrochemical or refining reactions carried out in the presence of catalysts previously contacted with a sulphur compound, exothermicity phenomena occur at temperatures frequently close to 150.degree. C. These exothermic phenomena are due to the conversion of oxides into sulphides or possibly oxysulphides into sulphides, which takes place at the time of starting up the reaction on introducing hydrogen onto the catalytic material.
These exothermic phenomena are not prejudicial in the case of small units, particularly when on starting the charge is in the liquid state, because the calories are easily evacuated when the charge passes into the gaseous state. The exothermic phenomena do not cause much of a problem if a refining reaction can take place in several beds of modest size in place of a single, large bed. However, it is becoming increasingly common to use large units (particularly residue treatment units), where the reactors can contain more than 200 tons of catalysts.
Generally, hitherto, after incorporating an adequate sulphur quantity into the catalytic material, i.e., a quantity able to bring about the stoichiometric conversion of oxides of active metals into sulphides, the following procedure is used with respect to the catalyst prior to the hydrogen reduction of said catalyst. The catalyst is fed into a reactor, purging takes place several times under nitrogen and, when there are no longer any oxygen traces in the enclosure, there is a passage into a hydrogen atmosphere and the heating furnaces are ignited, followed by heating up to 150.degree. C.
It is then possible to carry out the real sulphurizatio of the catalyst by introducing hydrogen. This exothermic reaction can be written in the following way, symbolizing by S the random sulphurizing agent, e.g., elementary sulphur, organic polysulphide, thiol, etc. EQU MoO.sub.3 +2S+3 H.sub.2 .fwdarw.MoS.sub.2 +3 H.sub.2 O
This reaction can give rise to a significant temperature elevation in the catalytic bed, which may lead to a damage to the catalyst or equipment.
When it is not possible to control an exothermic situation , there must be an emergency shutdown of the unit and all the valves must be opened in order to torch the effluents, with decompression producing cold, so that there is a return to normal atmospheric conditions.